This invention relates to a medium characterization system using reflected ultrasonic waves wherein attenuation data from the medium is measured by sending and receiving ultrasonic waves from an inhomogeneous medium, such as biological tissue. The invention particularly relates to a medium characterization system using reflected ultrasonic waves wherein the influence on the received signal, generated by different transmitting fields, corresponding to the sending frequencies, can be eliminated during measurement by sending ultrasonic waves having different frequencies.
In the field of diagnostic inspection of human bodies and flaw detection of metallic materials the characterization of a medium by transmitting and receiving ultrasonic waves is widely used. Particularly, as a system used for diagonstic inspection of the human body, an ultrasonic wave diagnostic system is already in practical use, but such as ultrasonic wave diagnostic system displays only the received signal which is reflected from a heteroacoustic point of a tissue at a location which depends on the elapsed travel time of the wave from the ultrasonic wave transmitting point. Recently, it has been attempted to measure the quality of tissues by analyzing this reflected wave and it is also expected that such a diagnostic system is put into practical use in view of further raising the diagnostic effect.
For example, Japanese laid-open patents Nos. 49-25785 and 49-38490 disclose systems wherein ultrasonic waves of two different kinds of frequencies are transmitted into a medium, the reflected waves of respective frequencies are received, and attenuation data for the medium can be measured from a ratio of the amplitude of the received signals to their respective frequencies. According to such disclosed techniques, the influence on the reflection rate caused by the angle and smoothness of the surface of the heteroacoustic point is eliminated. Data indicating only the attenuation of the ultrasonic waves caused by the internal tissues of the medium can be extracted from the waves reflected by the heteroacoustic point, and a qualitative diagnosis can be made of the medium tissues. Namely, the characteristics of a tissue can be discriminated and the quality of such tissue can be diagnosed.
On the other hand, the technique disclosed above also teaches that an error in measurement is easily generated when the intensity of the reflected wave is weak. Therefore the inventor of the present invention has developed a technique of measuring attenuation data for a medium by comparing the shape of the intensity spectrum of the transmitted wave and the received wave. This technique is presented in U.S. Pat. No. 4,452,082, corresponding to Japanese patent application No. 56-65536.
However, the dislcosed techniques indicate that they are based on the assumption that the transmitted ultrasonic wave beam has a one dimensional structure and is one-dimensionally propagated. Other improvements are necessary for an ultrasonic wave beam which has a three-dimensional structure and propagates three-dimensionally.
Namely, the profile of an ultrasonic wave beam which has a three-dimensional structure and which propagates three-dimensionally changes depending on the transmitting frequency, and the effective measuring volume included in the beam also changes depending on the frequency. Therefore, when using a transmitting wave having a plurality of frequencies, the influence of the medium in which the waves travel on the different frequencies of received signals is different and depends on the transmitting frequency according to the effective measuring volume. The received wave includes variations due to the frequency dependent effective measuring volume and variations due to the frequency dependency of the medium itself. Thus, it is necessary to be sure that the beam of transmitted ultrasonic waves is not dependent on frequency even when the transmitting frequency changes.
With this background, a method where the diameter of the ultrasonic wave transducer aperture is changed in accordance with the transmitting frequency and the ultrasonic wave beam of each transmitting frequency has the same three-dimensional characteristics is presented in U.S. Pat. No. 4,459,853, corresponding to Japanese Patent Application No. 56-48275, "Measuring System Using Ultrasonic Wave". A method where the reflected waves are selectively received only from the overlapped area of each transmitting frequency is presented in Japanese Laid-Open Patent No. 57-191546, corresponding to Japanese Patent Application No. 56-76986, "Ultrasonic Wave Measuring System." According to these patent applications, it is possible to achieve substantial frequency independence of the beam shape because the real acoustic field can be made frequency independent by controlling the transmitting field and/or the receiving field.
If the real acoustic field can be made frequency independent by controlling the transmitting field, (for example, in a reference medium such as water, which has approximately the same sound velocity as the measuring medium, is homogeneous and has no or weak frequency dependent attenuation) the frequency component (for example, spectrum shape) of the received echo signals from the various depths of the acoustic field must be similar to each other.
However, when there is a small error in the control of the transmitting acoustic field and there are many frequency components to be considered, e.g., a continuous spectrum, it is very difficult to make the real acoustic field frequency independent. Moreover, it is also difficult to cover both near and far acoustic fields with a single transmission control and even if both near and far acoustic fields are covered by two types of transmission control, it is still difficult to make the real acoustic field frequency independent in the transition range between the near and far acoustic fields.
For this reason, the frequency component data, for example, the spectrum distribution shape, at each depth in the acoustic field is no longer similar to the data of other depths, even in the reference medium.
In the case of a method allowing for the frequency dependence in the transmitting acoustic field and eliminating the dependency of the real acoustic field by control of the receiving acoustic field the similarity of the frequency component data of reflected waves from each depth in the acoustic field, for example, spectrum distribution shape, is particularly deteriorated. In other words, the acoustic field formed by transducers having the same aperture diameter has a divergence angle inversely proportional to the wavelength of the ultrasonic waves in the far field, and therefore, the ratio of the high to the low frequency components is lowered because the low frequency element diverges into a wider solid angle than that of the high frequency element as the depth increases. Even if the ultrasonic waves are reflected from the frequency independent reflector placed at each depth, the shape of the spectrum distribution of the waves reflected from each depth is no longer similar even in the reference medium.
As explained above it is actually impossible to keep the spectrum distribution shape unchanged for different depths of the reflection point. This is so even when control of the transmitting acoustic field and receiving acoustic field is carried out, and even when the measurement is carried out in a medium which is homogeneous, has almost the same sound velocity and has no or very weak frequency dependent attenuation, such as a reference medium of, for example, water. For this reason, there is a problem that the characterization of the measuring medium is also influenced by changes in the spectrum shape, depending on the depth of such reflecting point, and it is not possible to separate such changes from that of the frequency dependency of the measuring medium itself.